The capacitance and leakage rate of an ultracapacitor will vary from its rated value due to variation in the manufacturing process. Aging will also affect the capacitance and leakage of each individual cell. Because ultracapacitors are often charged and discharged as a series string and because the leakage and capacitance varies from cell to cell, the voltage of the individual ultracapacitors can vary from cell to cell. This cell voltage variation is disadvantageous because more energy will be stored in a series string when each cell is charged to its maximum operational voltage. Additionally, series connected cells are more likely to be overcharged which will prematurely age and rapidly degrade cells. The energy stored in an ideal capacitor is characterized by the following equation:
  E  =            1      2        ·          CV      2      
Typically, the prior art attempts to balance cells so that none of them become over charged. Overcharging would result in premature aging. Four prior art charging methods are common:
1. Distinct Voltage Drain Circuit—A circuit by which current is drained when the cell reaches a particular voltage. A circuit is connected over each cell.
2. Cell Compare and Drain—A circuit by which the voltages of two cells in a series string are compared and the cell and current is drained through a resistor and transistor from the cell with the higher voltage.
3. Resistive—A resistor is place over each cell. The resistor has a value that yields a current significantly higher than the leakage current of the cell. Because current is proportional to voltage, current from the cells with higher voltage is higher and therefore the cells tend toward having the same voltage.
4. Zener—This method is very similar to “Drain Circuit.” A zener diode, which conducts at a particular voltage, is placed over each cell. Unlike the “Drain Circuit,” zeners do not have a distinct conduction point and therefore begin conducting before the desired maximum cell voltage.